Mine's currently being replaced at the dealership - 68k miles.
They are due to be changed based on the OEM Maintenance Schedule at 60k miles.
as previously mentioned, these are interference engines (meaning that if the timing belt breaks, so will your valves when the piston head hits them.
It is highly recommended that you change the timing belt on schedule unless you actually enjoy doing valve work (or paying to have it done for you). ^^
A few good indications that its ready to be replaced are sputtering, noticable loss of power across the full rpm range (a symptom that the belt may have jumped a cog) or possibly a loud clattering from under the hood followed very shortly by stalling and inability to restart the engine. =p